1. Field of the Invention
The present invention relates to a method for manufacturing a group III nitride compound semiconductor. Especially, the present invention relates to a method for manufacturing a group III nitride compound semiconductor in which an epitaxial lateral overgrowth (ELO) method is used to form a layer on a substrate. The present invention also relates to a light-emitting device using a group III nitride compound semiconductor formed on a group III nitride compound semiconductor layer using the ELO method. A group III nitride compound semiconductor can be made of binary compounds such as AlN, GaN or InN, ternary compounds such as AlxGa1xe2x88x92xN, AlxIn1xe2x88x92xN or GaxIn1xe2x88x92xN (0 less than x less than 1), or quaternary compounds AlxGayIn1xe2x88x92xxe2x88x92yN (0 less than x less than 1, 0 less than y less than 1, 0 less than x+y less than 1), that is, those are represented by a general formula AlxGayIn1xe2x88x92xxe2x88x92yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61).
2. Description of the Related Art
A group III nitride compound semiconductor is a direct-transition-type semiconductor having a wide emission spectrum range from ultraviolet to red, and is applied to light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs). The group III nitride compound semiconductor is, in general, formed on a sapphire substrate.
However, in the above-described conventional technique, when a layer of a group III nitride compound semiconductor is formed on a sapphire substrate, cracks and/or warpage are generated in the semiconductor layer due to a difference in thermal expansion coefficient between sapphire and the group III nitride compound semiconductor, and dislocations are generated in the semiconductor layer due to misfit, which result in degraded device characteristics. Especially, dislocations due to misfit are feedthrough dislocations which penetrate the semiconductor layer in longitudinal direction, resulting in propagation of about 109 cmxe2x88x922 of dislocation in the group III nitride compound semiconductor.
FIG. 6 illustrates a schematic view showing a structure of a conventional group III nitride compound semiconductor. In FIG. 6, a buffer layer 2 and a group III nitride compound semiconductor layer 3 are formed successively on a substrate 1. In general, the substrate 1 and the buffer layer 2 are made of sapphire and aluminum nitride (AlN), respectively. Although the AlN buffer layer 2 is formed to relax misfit between the sapphire substrate 1 and the group III nitride compound semiconductor layer 3, possibility of generating dislocations cannot be 0. Feedthrough dislocations 4 are propagated from dislocation generating points 40 in longitudinal direction (a direction vertical to a surface of the substrate), penetrating the buffer layer 2 and the group III nitride compound semiconductor layer 3. Thus, manufacturing a semiconductor device by laminating various group III nitride compound semiconductor layers on the group III nitride compound semiconductor layer 3 results in propagating feedthrough dislocations 4 from dislocation generating points 41 which reach the surface of the group III nitride compound semiconductor layer 3, further through the semiconductor device in longitudinal direction. Accordingly, it had been difficult to prevent dislocations from propagating in the semiconductor device at the time when a group III nitride compound semiconductor layer is formed.
Accordingly, in light of the above problems, an object of the present invention is to realize an efficient method capable of forming a layer of a group III nitride compound semiconductor without generation of cracks and dislocations to thereby improve device characteristics.
In order to solve the above problems, the present invention has a first feature that resides in a method for manufacturing a group III nitride compound semiconductor, which hardly grows epitaxially on a substrate, by crystal growth, comprising: forming a buffer layer on a substrate into an island pattern such as a dot pattern, a striped pattern, or a grid pattern such that substrate-exposed portions are formed in a scattered manner; and forming a group III nitride compound semiconductor layer on the buffer layer by growing a group III nitride compound epitaxially in longitudinal and lateral directions.
Here forming substrate-exposed portions in a scattered manner does not necessarily represent the condition that each substrate-exposed portions is completely separated, but represents the condition that the buffer layer exists around arbitrary substrate-exposed portions. In order to form the buffer layer into an island pattern such as a dot pattern, a striped pattern or a grid pattern, the following method can be applied: forming the buffer layer on the entire surface of the substrate and then removing the desired portions of the buffer layer by etching; or forming a selective mask such as an SiO2 film on the substrate and partially forming the buffer layer.
The xe2x80x9clateralxe2x80x9d direction as used in the specification refers to a direction parallel to a surface of the substrate. By using the above-described method, the group III nitride compound semiconductor grows on the buffer layer in a longitudinal direction. The group III nitride compound semiconductor which grows on the buffer layer in a longitudinal direction also grows in a lateral direction in order to cover the substrate-exposed portions. The growth velocity of the group III nitride compound semiconductor in the longitudinal and lateral directions can be controlled by conditions of, for example, temperature, pressure, or supplying conditions of source materials. Accordingly, a group III nitride compound semiconductor layer reunited into one layer can cover the substrate-exposed portions which are not covered by the buffer layer from a base of the buffer layer which is formed into an island pattern such as a dot pattern, a striped pattern or a grid pattern. As a result, feedthrough dislocations of the group III nitride compound semiconductor exists only in the regions of group III nitride compound semiconductor layer formed on the buffer layer, which is formed into an island pattern such as a dot pattern, a striped pattern or a grid pattern. This is because feedthrough dislocations are not generated when the group III nitride compound semiconductor grows in a lateral direction but are generated when it grows in a longitudinal direction. Accordingly, surface density of longitudinal feedthrough dislocations of the group III nitride compound semiconductor layer decreases, and crystallinity of the device is improved. When a group III nitride compound semiconductor device which is manufactured using only a group III nitride compound semiconductor layer which is formed on the substrate-exposed portions, or the regions which are not covered by a buffer layer, surface density of feedthrough dislocations of the device can become 0.
The second feature of the present invention is a method for manufacturing a group III nitride compound semiconductor, which hardly grows epitaxially on a substrate, by crystal growth, comprising: forming a buffer layer on a substrate into an island pattern such as a dot pattern, a striped pattern, or a grid pattern such that substrate-exposed portions are formed in a scattered manner; forming a group III nitride compound semiconductor layer on the buffer layer by growing a group III nitride compound epitaxially in longitudinal and lateral directions; etching at least one of the regions of the group III nitride compound semiconductor layer, growing in a longitudinal direction on the buffer layer which is formed into an island pattern; and growing the group III nitride compound semiconductor, which is left without being etched, in a lateral direction. Forming substrate-exposed portions in a scattered manner is explained in the first feature.
In the second feature of the present invention, the group III nitride compound semiconductor layer is etched after carrying out the method of the first feature, and then it is grown in a lateral direction in order to cover the etched regions. As described in the first feature of the present invention, the surface density of longitudinal feedthrough dislocations of the group III nitride compound semiconductor layer decreases, and crystallinity of the device is thus improved. By etching the regions of the group III nitride compound semiconductor layer which grow on the buffer layer in a longitudinal direction and have feedthrough dislocations, feedthrough dislocations generated by the longitudinal growth of the semiconductor layer can be eliminated. It is preferable to also etch the buffer layer to expose the substrate during etching of the group III nitride compound semiconductor layer.
By growing the group III nitride compound semiconductor in a lateral direction again, the substrate-exposed regions can be covered and a group III nitride compound semiconductor layer which is reunited into one layer can be obtained. The lateral growth can be promoted by the conditions of, for example, temperature, pressure, or supplying conditions of source materials. As a result, feedthrough dislocations existing in the group III nitride compound semiconductor layer can be eliminated. Thus, the group III nitride compound semiconductor layer does not have longitudinal feedthrough dislocations, and crystallinity of the device is, therefore, improved. The scope of the present invention also involves a method of etching regions of the group III nitride compound semiconductor layer, including the upper surface of the buffer layer, wider than the width of the buffer layer in case that feedthrough dislocations are generated partially inclined (in a lateral direction). Similarly, all the regions of the group III nitride compound semiconductor layer which have feedthrough dislocations and grow on the buffer layer, which is formed into an island pattern such as a dot pattern, a striped pattern, or a grid pattern, is not necessarily etched. The group III nitride compound semiconductor layer can be reunited into one layer by growing it in a lateral direction again, even when feedthrough dislocations are left without being etched in the semiconductor layer. The scope of the present invention also involves a method of dividing etching and epitaxial lateral overgrowth (ELO) components into several parts, according to a position or a design of the regions to form the buffer layer and limitations in the process afterward.
The third feature of the present invention is to combine epitaxial growth of a group III nitride compound semiconductor layer formed on the buffer layer in longitudinal direction and epitaxial growth of the group III nitride compound in a lateral direction by using the difference between the velocities of epitaxial growth of the group III nitride compound semiconductor layer on the buffer layer and on the exposed substrate, in order to obtain a group III nitride compound semiconductor layer which covers the surface of the substrate. The difference between the velocities of epitaxial growth of the group III nitride compound semiconductor on the buffer layer and on the substrate can be easily controlled by the conditions of, for example, temperature, pressure, or supplying conditions of source materials. Similarly, the velocities of growing the group III nitride compound semiconductor epitaxially on the buffer layer in longitudinal and lateral directions can be controlled. By controlling these conditions, surface density of feedthrough dislocations of the group III nitride compound semiconductor layer in longitudinal direction is decreased, and crystallinity of the device is improved.
In this feature, the difference between the velocities of epitaxial growth of the group III nitride compound semiconductor on the buffer layer and on the substrate is used. But this does not necessarily exclude the possibility that the group III nitride compound semiconductor comprises a compound which is identical to that of the buffer layer in view of stoichiometric composition. Depending on the layer formed under the semiconductor layer and the condition of the epitaxial growth, the compound of the group III nitride compound semiconductor can be non-crystalline, a set of micro-crystalline and polycrystalline, or single-crystalline. The difference between the velocities of epitaxial growth of the group III nitride compound semiconductor layer on the buffer layer and on the exposed substrate is an essential point of the present invention, and combination of the group III nitride compound, the compound forming the buffer layer, and materials to form the substrate is a method of generating the different epitaxial growth velocities.
The fourth feature of the present invention is that the substrate is made of sapphire. As a result, it is difficult to epitaxially grow the group III nitride compound semiconductor on the substrate.
The fifth feature of the present invention is that the buffer layer is made of aluminum nitride (AlN). And the sixth feature of the present invention is that the group III nitride compound semiconductor which grows in lateral direction does not comprise aluminum (Al).
The seventh feature of the present invention is to obtain a light-emitting group III nitride compound semiconductor device by forming an another group III nitride compound semiconductor layer on the group III nitride compound semiconductor, which is formed on the region where the buffer layer is not formed, by using the above-described method. Because the group III nitride compound semiconductor layer is laminated on the regions which have no surface density of longitudinal feedthrough dislocations, reliability of the device is improved.
The eighth feature of the present invention is to obtain only the group III nitride compound semiconductor layer by using the above-described method. The group III nitride compound semiconductor layer, which is a layer laminated on the substrate, is left by removing the substrate. If necessary, the buffer layer can be removed with the substrate. When an insulator is used to form the substrate, the group III nitride compound semiconductor layer can dope an arbitrary dopant into the substrate so that the substrate has an arbitrary resistivity, and the group III nitride compound semiconductor layer with non-insulated substrate can be obtained. Because the group III nitride compound semiconductor layer has little or no surface density of feedthrough dislocations, it can be useful as a semiconductor substrate.